Has the strain Kurt experiences when using his powers to a high degree ever been completely defined? I understand that it's potentially lethal, but that it's also something the body can get used to, right?
Is it simply a matter of his body using up a lot of energy so he's spent? Does his blood pressure go through the roof?
When Rogue used his powers to teleport Nimrod's arm off, didn't she say the strain would have ripped her apart if she hadn't absorbed Colossus' metal form? I seem to recall Kurt describing it that way at some other point too - that it almost ripped him apart. Does that mean that it's an actual force tearing through him and not just him spending energy? What kind of physical damage does it create? Can it cause internal bleeding? Is it comparable to any strain or stress a human body can experience in real life?

Does anyone know or can anyone make a good guess? Has this been answered somewhere?

I can honestly say I have no real theories on the subject. I don't believe it has ever been defined; all I know is Kurt only feels a great deal of fatigue after a large display of power. I think of it as lifting weights. Small weights can be lifted easily and with little strain. Heavy weights require much effort and concentration and the strain is great. But this doesn't take any noticable toll on the body besides exhaustion. Now if you over do it, I suppose you could rip something but no such instance has occured with a teleport. Also, Kurt has been Bamfing since his teens; comparable to a professional body builder. If you get someone not accustomed to the force acting upon them, of course they wouldn't be able to handle it.

Originally posted by Bamfing_Bob
I can honestly say I have no real theories on the subject. I don't believe it has ever been defined; all I know is Kurt only feels a great deal of fatigue after a large display of power. I think of it as lifting weights. Small weights can be lifted easily and with little strain. Heavy weights require much effort and concentration and the strain is great. But this doesn't take any noticable toll on the body besides exhaustion. Now if you over do it, I suppose you could rip something but no such instance has occured with a teleport. Also, Kurt has been Bamfing since his teens; comparable to a professional body builder. If you get someone not accustomed to the force acting upon them, of course they wouldn't be able to handle it.

I really wish they would define it better, though. One would think Hank would be all over him, having him teleport with an x-ray device or something, to find out the nature of the strain, at least out of scientific curiosity. As it is, it's so...vague. It's supposed to be something that can kill Kurt, but too often Kurt seems to be fine after just some rest. Take the instance in the First Foursaken arc of Uncanny as an example - Kurt teleports and the Watcher's presence amplifies the distance of the teleport, causing Kurt to collapse. Betsy remarks how the strain "almost killed" him. However, he doesn't exactly LOOK like someone who just almost died and after some rest, he's his usual sunny self. If Kurt can heal up that quickly from it, I'm not sure I feel the threat of the strain much.

"If you live your life to please everyone else, you will continue to feel frustrated and powerless. This is because what others want may not be good for you. You are not being mean when you say NO to unreasonable demands or when you express your ideas, feelings, and opinions, even if they differ from those of others.â€

Originally posted by JSherlock
(...)
Basically boiled down - fatigue exists so we don't kill ourselves - but if we're at the point of fatgiue we could die, since exaustion and fatigue are telling us to stop before it's too late.

I can't tell you why he becomes tired, but I can tell you why people become sick after teleporting with him...

Kurt's teleports have the strong smell of fire and brimstone. Brimstone (or sulfur) has a smell reminicent to rotting eggs. When sulfur burns, the scent becomes extremly unbearable to the nasal passages and triggers a near vomitting reaction to the body and causes physical weariness. Burning sulfur can also kill you through asphyxiation(choking).
Why the scent does not affect Nightie: 1) He's gotten used to it over the years 2) IDK...

I always assumed it had something to do with passing through another dimension, to which he was accustomed or even attracted (like, um, gravity or magnetism attracts), but that wouldn't "click" with anyone else. Of course, I stink at physics.

Forgot if they mentioned it in The Science Of The X-Men, but I think they did. Durn it. I read the Crawler section several times, too. But I've blinked since then.

"Humanity is a parade of fools, and I am at the front of it, twirling a baton." From Chapter 9 of _Brother Odd_ by Dean Koontz / from Chapter 10: "Life you can evade; death you cannot."

It makes sense Kurt is exhausting himself since he's the one opening the portal (2 portals actually since there's one to leave and one to come back), but the strain that others experience must be from actually passing through the portal(s).

I've always thought that the stress that Kurt's passengers experience is a kind of physical pressure and disorientation. If you ride with Kurt, you're actually squeezing through a hole in the universe into another dimension, traveling an undefined distance, and then squeezing back in. The closest real-world comparison I could imagine is what deep-sea divers go through when they experience extreme pressure changes, or when astronauts and fighter pilots experience strong g-forces.